Gamasutra has word from Blizzard's Frank Pierce, who says that their upcoming Diablo III may cannibalize World of Warcraft before the release of Titan, their next MMORPG. "It's hard to say. I think even a shorter-term concern is whether or not we might see cannibalization of WoW players from Diablo III when we launch it, because it's a similar type of experience. Not exactly similar, but it's that RPG feel," he tells them in response to a question about Titan cutting into WoW. "For us, I think it's really important that we recognize that somewhere, sometime it's likely that someone is going to cannibalize World of Warcraft players. So it's better we cannibalize them ourselves than let someone else do that, because if we cannibalize them ourselves, they're still a Blizzard customer."

Odd and copious use of the word cannibalize. Customers are not to be eaten, find better PR phrasing, douche.

WoW and D3 will be similar only in that there are talents, abilities, leveling, etc... all standard fair rpg shit. So that's obvious. By the time D3 comes out in 2017, my guess is there will be less people transferring from WoW and more people who don't play WoW at all or have since retired it.